Spectroscopy, such as rotational spectroscopy, is a powerful structural tool in physical chemistry. For example, the relationship between a molecular structure and its rotational transition frequencies can be used for structure determination of gas phase samples. Other effects in the rotational motion of molecules, such as centrifugal distortion, hyperfine spectral structure from quadrupolar nuclei, or frequency shifts caused by tunneling motion, can be used to provide further characterization of the molecular structure and low frequency vibrational motions.
The millimeter (mm) wave or terahertz (THz) frequency region is a particularly useful region for chemical detection and characterization. For room temperature samples, this is the region where pure rotational spectra are most intense. All molecules with a permanent dipole moment have a pure rotational spectrum. Such a spectrum includes a large number of sharp transitions that can serve as a “fingerprint” of a molecular identity. For example, such transitions can be about 1 megahertz (MHz) in width at about 300 gigahertz (GHz) in a low-pressure gas cell. In one approach, the transition frequencies can be related back to the molecular identity, such as through fitting with a molecular Hamiltonian, which can explain measured transition frequencies and relative intensities in terms of calculable physical parameters. From the intensities of the transitions observed by millimeter- and sub-millimeter-wave spectroscopy, absolute molecular concentrations can be derived with high accuracy and selectivity.